Over the years much development has taken place in the manufacture of electric motors to increase the operating characteristics and their efficiency and also to reduce their size and cost. Recent developments in motor technology include the variable reluctance motor that first became widely known in the early 1980's. This is an electric motor in which torque is produced by the tendency of its movable part to move to a position where the inductance of the excited winding is at a maximum. The windings of the various phases are typically excited when the inductance is increasing and are unexcited when the inductance is decreasing, for purposes of motoring operation. Two types of reluctance motors may be distinguished, including the switched reluctance motor, sometimes also referred to as a variable reluctance motor, in which excitation comprises a sequence of current pulses applied to each phase in turn, and synchronous reluctance motors in which excitation comprises a set of polyphase balanced sine wave currents.
In spite of the developments in the motor industry, various shortcomings may be distinguished in existing electric motors.
In U.S. Pat. No. 3,060,337 to Henry-Baudot, an electric motor is disclosed in which the windings are produced by making use of printed circuit techniques. However, the limited number of poles increases the amount of cogging and reduces the efficiency by increasing the amount of eddy currents due to the increased cross-sectional area of the material. Many other motors use even fewer poles thereby exacerbating the problems.
U.S. Pat. No. 4,260,926 to Jarret et al. discloses an embodiment using 48 stator poles and 58 rotor poles. While addressing some of the problems mentioned above, it nevertheless fails to address the possibility of providing a variable torque facility since the poles are driven into saturation. Furthermore the number of poles are limited by the physical construction of the poles.
U.S. Pat. No. 3,887,854 to Parks defines a variable speed motor. It however, does not have the facility for varying the torque. Clearly it would be desirable to have a motor displaying both variable speed and variable torque characteristics.
The use of amorphous materials for the stator body has been considered in a number of patents including U.S. Pat. No. 4,578,610 to Kliman et al. in which an amorphous metal tape is wound in an annular fashion. Poles are formed in the stator by machining slots into it. The rotor, in turn, is provided with a plurality of equally spaced permanent magnets. Another application of the use of an amorphous metal alloy is that described in U.S. Pat. No. 5,028,830 to Mas wherein an amorphous metal alloy strip is wound about a hub. In both of the above patents the poles are mechanically formed, thus limiting the number of poles that can be formed on the stator.
Developments have also been made to reduce the thickness of motors by making use of a planar construction as discussed in U.S. Pat. No. 4,922,145 to Shtipelman. The rotor in this patent, however, includes a plurality of discrete permanent magnets, thereby once again limiting the number of poles that can be formed on the rotor. The stator comprises two plates one on either side of the rotor disk, the poles being defined by 20 pairs of poles on each of the stator disks. While these are described as being formed either in a conventional manner or by using photofabrication, the poles of the stator extend radially and are limited in number by the magnets of the rotor. The concept of producing the windings using an etching process is also considered in U.S. Pat. No. 5,021,698 to Pullen et al. However, neither Pullen nor any of the other prior art references consider forming poles, as opposed to the windings, using a lithographic process.
Another development has been the concept of using flat motors which was considered in U.S. Pat. No. 5,144,183 to Farrenkopf. Switching, however, is achieved by means of a commutator, which again limits the number of stator poles that can be incorporated in the motor. Another micromotor is disclosed in U.S. Pat. No. 5,216,310 to Taghezout in which a magnetic field is established substantially parallel to the axis of rotation. The rotor, however, comprises a magnetized rotor producing a magnetic field. The use of magnets in the rotor once again limits the number of poles.
In U.S. Pat. No. 5,229,677 to Dade et al. a pancake configuration motor is disclosed with an axial air gap, thrust bearings maintaining the position of the rotor relative to the stators. Once again, the rotor includes a plurality of permanent magnets, which limit the number of rotor poles.
U.S. Pat. No. 5,412,265 to Sickafus discloses yet another planar micro-motor. This motor provides the added advantage of achieving speed and direction control by regulating the energizing of micro-coils on the stator. Once again, however, the poles of the rotor are envisaged to be permanent magnets or are made of a magnetically soft material such as iron. The use of such discrete rotor poles limits the number of poles that can be produced on a rotor disk.
As mentioned above, the limit in the number of poles leads to inefficiency insofar as the cross sectional area of each pole is relatively large thereby producing large eddie currents. Since the switching frequency is also reduced by the smaller number of poles, the skin depth of current flow increases, thereby requiring thicker rotors when using them for purposes of magnetized storage media such as hard disk drives. Furthermore, the smaller the number of poles, the greater the amount of cogging that takes place and the lower the accuracy with which the position of the rotor relative to the stator can be determined.